Content data distribution systems for delivering content data such as music data or movie data through the Internet is becoming popular, since the Internet and electronic commerce is becoming popular. In such content data distribution systems, a user decides to purchase content data based on motivations caused by various events. Above all, a user is likely to use so-called “word of mouth” from acquaintances or friends for reference. Therefore, it is desirable to promote sales of content data based on such “word of mouth”.